The present invention relates to pneumatic brake control systems for railway vehicles and particularly to the quick service function provided by such brake control systems.
Conventional ABD type brake control valve devices, as are typically employed on railroad freight cars, incorporate a quick service feature in which compressed air in the train brake pipe is vented locally at each car of the train when a brake application is initiated, in order to propagate the brake application signal through the train. A service piston of the control valve device is operable between brake release and service application positions in accordance with the predominance of brake pipe pressure on one side of the piston and auxiliary reservoir pressure on the opposite side. The service piston carries a graduating valve that is fixed with the piston for direct movement therewith and a slide valve that moves with the piston through a lost-motion connection. Accordingly, relative motion exists between the slide valve and graduating valve during initial piston movement in both the application and release directions.
There is less frictional resistance to movement of the graduated valve than the slide valve. The initial movement of the service piston toward application position in response to a reduction of brake pipe pressure therefore moves the graduating valve relative to the slide valve to establish a flow path via which brake pipe pressure is connected to a vented quick service bulb. This local venting of brake pipe pressure supplements the trainline reduction of brake pipe pressure to promote the further movement of the service piston and slide valve to service position, and also to accelerate the brake application signal through the train for faster brake response. In the ABD type valve, this is commonly referred to as "preliminary quick service."
A bias force in the form of a leaf spring acts laterally on the slide valve to provide face-to-face sealing pressure at the interface of the slide valve and its seat in the slide valve bushing. The slide valve is also pressed to its seat by air pressure. This combined spring load and air load on the slide valve causes frictional resistance that the service piston encounters upon engagement with the slide valve following the initial piston movement to preliminary quick service position. The resistance imparted by the slide valve, combined with the compressive force of the piston stabilizing spring, thus determines the application differential between brake pipe and auxiliary reservoir pressure that is required to move the service piston beyond quick service position to service application position.
It will be appreciated that the duration the service piston remains in quick service position, prior to moving the slide valve, has a direct influence on the degree of quick service activity, and, conversely, the effectiveness of quick service in locally reducing brake pipe pressure influences the time required to develop sufficient differential to move to service position and terminate quick service. Because of this self-regulating feature, quick service is automatically effective for different individual car lengths or volumes of brake pipe.
Because of manufacturing tolerances and other variables which influence the resistance to slide valve movement, the actual application differential and thus the duration the service piston remains in quick service position varies somewhat from car to car. Consequently, the optimum propagation rate of the brake application signal is not always realized.
Whenever the application differential increases sufficiently to overcome the piston resistance, the slide valve and graduating valve move in unison with the service piston to sequentially cut off the quick service flow path and to establish an application flow path between the auxiliary reservoir and brake cylinder. However, because of the close proximity of the porting in the piston slide valve, manufacturing tolerances can also cause a port overlap condition to arise in which the application flow path cracks open before the quick service flow path is fully cut off, thereby causing the piston to stall, particularly when the application differential acting on the service piston is relatively low. Such a condition further degrades the propagation rate of the brake application signal.
When it is desired to release the brake, the release differential required between brake pipe and auxiliary reservoir pressures is governed principally by the same frictional resistance to move the slide valve back.